Hi Roxanne,
I’m not aware of a way to keep calluses going… At least not in the way you would want for playing guitar.

If it were me I’d probably be trying to figure out a way to practice anyway… Perhaps with a portable guitar, or even just the guitar neck.

Are there any possibilities along that line? Obviously I don’t know why you are not going to be able to practice so I couldn’t offer any suggestions there.

Tomas